Digital to analog converters for use in video display systems comprise a plurality of output current units arranged to be switched on in response to a digital input signal and produce a required analog current output representing respectively red, green and blue signal levels. It is desirable that such digital to analog converters include current units which provide analog current output values accurately reflecting the digital input and maintain the required output currents with variation in the output voltage which may be needed to meet the compliance specification for the video DAC.
Known current units for use in such DACs have used a cascode circuit in which two transistors are connected in series between a voltage supply line and an output of the current unit, the two transistors being arranged to conduct in saturation so as to provide a very large output resistance and thereby current stability with variation in the output voltage. To control switching of such current units known systems have incorporated matched and complementary steering transistors acting as switches in series with the cascode transistors between the supply voltage line and the unit output. The switching transistors have been arranged to receive simultaneous complementary control signals from inverter circuitry so that only one of the steering transistors is switched on at any one time. This provides two complementary outputs from the unit one of which will give the required data values for use by the video display system. While the cascode transistors operate in saturation the steering transistors operate in their linear region.
In MOS technology and particularly in CMOS technology the supply voltages used are gradually being reduced thereby reducing the head room between a supply voltage VDD and a ground line. With the above mentioned prior art system using steering transistors in series with a cascode circuit between a supply voltage and ground potential problems can arise in providing sufficient voltage across the steering transistors when the supply voltage is reduced below 5 volts.
It is an object of the present invention to provide an improved output current unit using a cascode circuit which may be switched between on and off states and can maintain the cascode transistors in saturation with reduced supply voltage.
It is a further object of the invention to provide an improved current output unit in which cascode transistors remain in saturation while maintaining compliance with variation in output voltage.
It is a further object of the invention to provide improved inverter circuitry for controlling transistors in a cascode circuit.